Two types of image forming apparatus that use endless belts are used as an image forming unit. One of these apparatuses uses the belt as an intermediate transfer medium, and a plurality of image forming unit are placed around the belt and color toner images are transferred on top of each other directly onto the belt so as to form color toner images having either a plurality or a multiplicity of colors. Thereafter, the color toner images are transferred to a sheet shaped medium such as a paper. Thus this apparatus is known as an intermediate transfer type of color image forming apparatus.
The other apparatus uses the belt as a means for transporting the paper. This apparatus also has a plurality of image forming units placed around the belt, however, the paper is transferred together with the belt and a color image is obtained by sequentially transferring color toner images on top of each other onto the paper using the image forming unit during the transporting process. Therefore, this type of apparatus is known as a tandem type of image forming apparatus.
1. Intermediate Transfer Type Image Forming Apparatus
An example of an intermediate type color image forming apparatus is disclosed in Japanese Patent Application Laid-Open (JP-A) No. 10-177286. As shown in FIG. 17A, a belt 10 serving as an intermediate transfer medium is extended between two rollers 12 and 13 provided facing each other at a distance. The belt 10 is rotated by these rollers and a processing unit used for forming an image are placed around the belt 10.
If the direction in which the belt rotates is taken as that indicated by the arrow a, then a first image forming unit 14 and a second image forming unit 24 are provided beneath the belt 10 and between the roller 12 and the roller 13 as a processing unit which forms an image in the order given going from the upstream side in the direction of rotation of the belt. Moreover, a transfer roller 11 is provided so as to be able to be moved towards or away from the roller 13, and a cleaning blade 61a is provided so as to be able to moved towards or away from the roller 12.
The first image forming unit 14 is provided with a photoconductor drum 16 serving as an image carrier; not shown electrification unit placed around the photoconductor drum 16; not shown optical writing unit; a first developing apparatus 6 serving as a first developing unit comprising a A color developer 19 serving as a developing unit and a C color developer 20 serving as a developing unit; and a not shown cleaning unit.
The second image forming unit 24 is provided with a photoconductor drum 26 serving as an image carrier; not shown electrification unit placed around the photoconductor drum 26; not shown optical writing unit; a second developing apparatus 8 serving as a second developing unit comprising a B color developer 29 and a D color developer 30; and a not shown cleaning unit.
The image forming process is based on a typical electrostatic recording process, as will be noticed from the first image forming unit 14 and entails using optical writing unit to write an electrostatic latent image in a particular color onto a photoconductor drum that has been uniformly charged in darkness by an electrostatic unit, and then visualizing this electrostatic latent image using the first developing apparatus 6 and transferring the toner image onto the belt 16 (intermediate transfer).
Both the first developing apparatus 6 in the first image forming unit 14 and the second developing apparatus 8 in the first image forming unit 14 have the function of visualizing images each using toner of two different colors. Therefore, if black is added to the three primary colors to give four colors, then if these four colors are shared between each of the developers 19, 20, 29, and 30, it is possible to create a four color image.
Accordingly, if, while the same image formation area of the belt 10 is sequentially passing the two image forming apparatuses 14 and 24, as a result of transfer bias imparted by a first transfer brush 41 and a second transfer brush 42 serving as an intermediate transfer unit (a first transfer unit) that are provided facing the photoconductor drums 16 and 26 respectively with the belt 10 sandwiched between the respective brushes and photoconductor drums, a toner image is transferred in each color one by one on top of the other onto the belt 10 and, while the image formation area of the belt 10 onto which two colors have been transferred one on top of the other is sequentially passing the above two image forming unit 14 and 24 once again, toner images of different colors to the ones transferred in the previous transit are transferred in superposition by each of the image forming unit, then, at the point when the image formation area has passed twice over each of the image forming unit 14 and 24, it is possible to obtain a full color toner image by the superposed transfer onto the same image area.
The full color toner image is then transferred (i.e. the final transfer) onto paper P which is a sheet shaped medium. This transfer is performed by applying transfer bias to the transfer roller 10 used for the final transfer that has been placed in a state in which, at the time of transfer, it is rotated by pressure from the roller 13 below via the belt, and by passing the paper P through the nip portion between the transfer roller 11 and the belt 10. After the final transfer, the full color toner image carried on the paper P is fixed by a not shown fixing unit enabling a full color final image to be obtained on the paper P.
In this image forming process, using the position of the transfer roller 11, for example, as a reference, A color and B color toner images are transferred one on top of the other on the same image formation area of the belt 10 after the first rotation of the belt 10. Further, C color and D color toner images are further transferred one on top of the other on this same image formation area of the belt 10 after the second rotation of the belt 10. Thereafter, these four color superposed toner images are transferred onto the paper P.
When a four color superposed toner image is formed on the paper P and the four color superposed toner image arrives at the transfer roller 11, the transfer roller 11 need to be press contacted against the roller 13 in order to perform its transferring function. However, because it is necessary to allow the A color and B color superposed toner image to pass through without being damaged at all at the point when the A color and B color superposed toner image arrive, the transfer roller 11 is moved away from the roller 13 at this time. Therefore, the transfer roller 11 has such a construction that it can be moved towards or away from the roller 13 in the image forming process.
When the toner image is transferred onto the paper P by the transfer roller 11, residual toner remains on the belt 10. This residual toner contaminates the surface of the belt 10 and causes the images that are formed subsequently to be damaged. It is therefore necessary to remove this transfer residual toner prior to subsequent transfers by the image formation unit 14 and 24, and a cleaning unit is provided as this removal unit.
The cleaning blade 61a functions as the above-mentioned cleaning unit and it has such a construction that it can be moved towards or away from the roller 12 via the belt 10. This cleaning blade 61a is also controlled so as to be able to move towards or away from the belt 10 at the time when the A color and B color superposed toner image passes the position of the blade 61a such that the A color and B color superposed toner images formed during the first rotation of the belt 10 are not removed by cleaning. Immediately after the four color superposed toner image (i.e. the toner image formed from the A color, the B color, the C color, and the D color) is transferred onto the paper P, the blade 16a makes contact with the belt 10 and cleans it only when the relevant image formation area is passing the blade 61a in order for the transfer residual toner to be removed.
The cleaning blade 61a is moved towards or away from the roller 12 during the image formation process. The first image forming unit 14, the second image forming unit 24, the transfer roller 11, the cleaning blade 61a, the transfer brushes 41 and 42 and the like comprise the processing unit used for image formation provided around the belt 10.
In this type of intermediate transfer type of image forming apparatus, in order to increase the transfer accuracy of the transfer roller 11 acting as the final transfer unit, conventionally, a structure has been employed in which the roller 13, which can be moved towards or away from the transfer roller 11 via the belt 10, is used as the drive roller for the belt 10 and a drive source MO2 is linked to the roller 13.
The structure thus comprises the transfer roller 11 moving towards or away from the roller 13 via the belt 10 and the cleaning blade 61a moving towards or away from the roller 12 via the belt 10, and both of these impart a rotation load variation to their corresponding roller. However, if a comparison is made between the load variation affecting the roller 12 due to the movement of the cleaning blade 61a and the load variation affecting the roller 13 due to the movement of the transfer roller 11, then the load variation affecting the roller 12 is overwhelmingly greater. The reasons for this are because the transfer roller 11 has been designed so as to have reduced rotation resistance and to be freely rotatable when in contact, and because the impact at the time of contact is minimal due to highly elastic materials being used.
In contrast to this, due to its function, the cleaning blade 61a is positioned so as to be in contact with the belt at an angle whereby it tends to dig into the roller 12. Moreover, because a hard resin material is used due to its properties when scraping away the residual toner, the impact at the time of contact is large.
Therefore, if the cleaning blade 61a is moving relative to the belt 10 when the photoconductor drum 16 or the photoconductor drum 26 are transferring a toner image onto the belt 10, the roller 12, which is the slave roller, is directly affected by the variations in the load and, although only slight, unevenness occurs in the rotation thereby causing the tension on the belt 10 to vary.
On the other hand, because the rotation speeds of the photoconductor drums 16 and 26 are constant, the relative speed between the belt and the periphery of the photoconductor drum changes due to the variations in the belt tension, and it has been determined that color misregistration arises in the intermediate transfer image in the first image forming unit 14 and the second image forming unit 24 and pitch unevenness is generated.
With a tandem type belt, an extremely long circumference needs to be secured, however, the molding of the endless belt is prohibitively expensive. Therefore, normally, a sheet shaped endless belt is used and the two ends thereof are joined together by adhesive or the like to form a pseudo endless belt. However, during image formation, it is imperative that the joint be avoided (i.e. not be used).
The color image forming apparatus disclosed in JP-A No. 10-177286 has developing apparatuses 6 and 8 positioned around photoconductor drums 16 and 26, as shown in FIG. 17A, and toner of one color is supplied to the photoconductor drum for each revolution of the photoconductor drum so as to develop an electrostatic latent image which is then transferred onto the belt 10. The transferred toner image of the first color then has the toner image of the second color transferred in superposition on the first color toner image in the second rotation of the belt 10. The third and fourth color toner images are then transferred in the same way.
Thus, by sequentially transferring the toner images in the four colors on top of each other on the belt 10, a full color toner image is formed on the belt 10. Thereafter, processes to transfer and fix the toner image onto the paper P are performed. In an image forming apparatus that uses the belt 10 as an intermediate transfer body in this way, through holes and reflective marks and the like are provided in the vicinity of both edges in the transverse direction of the belt 10 and a transmission type or reflection type of photosensor is provided on the image forming apparatus body or on the belt unit for detecting the holes or reflective marks. The timing at which the image is then written onto the photoconductor drum is then controlled on the basis of the detected timing.
A further reflection type of photosensor is also provided for detecting the density of the toner transferred onto the belt 10. Process controls such as electrostatic bias control and transfer bias control are then performed on the basis of the signal levels of the toner density pattern formed (i.e. of the toner density) for each color. Typical examples of this intermediate transfer belt mark (or hole) sensing are the technologies disclosed in JP-A Nos. 5-35124, 9-54476, 9-106199, 9-96943, 7-036249, 11-249526, 11-160928, 11-65397, and 11-223976. Moreover, a typical example of the toner density sensing method is the technology disclosed in JP-A No. 9-304997.
In the image forming apparatus, as explained above, that uses a belt as the intermediate transfer body, a toner density detection unit for process control and a belt mark detection unit for a combination of at least four colors are provided for the belt, and the accuracy and stability of the detection are among the most important factors affecting the image quality. Therefore, the detection needs to be performed with a high level of accuracy. However, the detection position, namely, the position of the photosensor for a belt in the conventional format, as can be seen in the conventional example, is located for a particular reason at the outer peripheral circumference of belt support rollers positioned facing each other so as to support the belt. In some cases this position is at substantially the central area between the support rollers, however, in the majority of typical apparatuses, the position where the photosensor is located is not given a great deal of consideration and it is generally fit into any space leftover as a result of the structural layout of the apparatus.
However, if no consideration is given to the location of the photosensor because precedence is being given to the layout, there is a risk that the accuracy of the detection will be deleteriously affected. For example, it is not preferable for the photosensor to be placed near the developer where it is most likely to be affected by splashes and spillages of toner, or for the photosensor to be located facing upwards even if it is not placed close to the developer as these locations are affected by toner contamination (i.e. by toner adhering to the light emitting and light receiving surfaces of the photosensor).
If the photosensor is placed at a position away from the support roller, then vibration when the belt is being driven and slackness increase the further the belt is located away from the belt support rollers. In particular, because, marked vibration is generated in the extended surface on the slack side, the accuracy when using an optical detection method whose depth range is narrowly limited, such as is the case with a photosensor, is reduced.
The technology disclosed in JP-A No. 11-223976 is intended to provide a technology for solving the above problem, however, a special part known as a backing member is required. Moreover, in the technology disclosed in JP-A No. 9-54476, the outer peripheral surface of the belt support rollers is used, however, an extremely high degree of accuracy is required in the positioning of the photosensor and the roller when performing detection at the curvature position and if there is even a slight amount of mispositioning, the fear exists that the detection accuracy and stability will be reduced.
As shown in FIG. 17A, in the color image forming apparatus disclosed in JP-A No. 10-177286, two image forming units of same shape are provided below the belt 10. The extended surface of the belt 10 facing these image forming units become the tensioned side surfaces when the belt 10 is driven. Moreover, during the first transfer, the belt 10 that has been moved away by the transfer brushes 41 and 42, which are provided with approach/separation mechanisms, is made to approach the photoconductor drums 16 and 26.
In order to increase the transfer efficiency, it is necessary to bend the belt 10 using the transfer brush rollers 41 and 42 and to sufficiently press the photoconductor drums 16 and 26 so as to obtain the contact width between the photoconductor drums 16 and 26 and the belt 10. In other words, during intermediate transfer, force to make the transfer brushes bend the belt 10 and force to press the photoconductor drums 16 and 26 are necessary.
Therefore, in a structure in which the bottom side of the belt is made the tensioned side extended surface, because it is necessary for the contact to be maintained and not be pushed backwards by the tension in the belt, even greater force is necessary. Moreover, because the structure uses a plurality of image forming units, namely, the first image forming unit 14 and the second image forming unit 24, and because transfer brushes are provided in each image forming unit, considerable force is needed for moving the transfer brushes at the extended surface on the tensioned side of the belt 10. Alternatively, the fear arises that the transfer will be poor because of the narrow transfer width.
2. Tandem Type Image Forming Apparatuses
Tandem type image forming apparatuses such as that shown in FIG. 17B are also known. In FIG. 17B, the belt 10′ having a holding function of holding paper is extended between rollers 12′ and 13′, which serve as support members, facing in a horizontal direction.
Photoconductor drums 71Y, 71M, 71C, and 71BK, which serve as image carrying bodies for carrying toner images in each of Y (yellow), M (magenta), C (cyan), and BK (black) are arranged in a row adjacent to the belt 10′ in the above order from the upstream side in the direction of rotation of the upper belt of the belt 10′ as shown by the arrow.
Around each of the photoconductor drums 71Y, 71M, 71C, and 71BK, non-contact type electrostatic devices 72Y, 72M, 72C, and 72BK, that use corona discharge wire, cleaning units 1Y, 1M, 1C, and 1BK, and the like are provided in the above order in the rotation direction. Developing rollers 4a provided for each developing apparatus 74Y, 74M, 74C, and 74BK are arranged adjacent to the corresponding photoconductor drum.
The image forming apparatus is formed from the respective photoconductor drums and the electrostatic devices, developing apparatuses, cleaning units, and the like arranged around the photoconductor drums. In other words, image forming units 14BK′, 14C′, 14M′, and 14Y′ are arranged in that order facing the belt 10 as means for forming images using the respective colors Y, M, C, and BK.
The non-contact type transfer apparatuses 73Y, 73M, 73C, and 73BK which use discharge wire via the belt 10′ are provided facing the photoconductor drums 71Y, 71M, 71C, and 71BK in the image forming units 14BK′, 14C′, 14M′, and 14Y′.
Moreover, writing unit 18′ is provided above the photoconductor drums 71Y, 71M, 71C, and 71BK. Exposure light Lb that has been modulated in accordance with color image signals is emitted and irradiated onto an exposure section between the developing apparatus and the electrostatic apparatus in each photoconductor drum 71Y, 71M, 71C, and 71BK.
The belt 10′ is driven to rotate in the counter clockwise direction, as shown by the arrow. A pair of resistance rollers 75 are provided at a position further upstream than the upstream end of the upper belt of the belt 10′. The paper P is fed by a supply roller 76 towards the resistance rollers 75.
A fixing apparatus 50′ is provided at a position further downstream than the downstream end of the upper belt of the belt 10′. A non-contact type of static electrifier 78 that uses corona discharge wire is provided as a paper suction unit above the roller 12′ supporting the belt 10′ at the upstream end portion of the upper belt of the belt 10′ such that paper is electrostatically suctioned to the belt 10′. A removal unit 79 for deelectrifying the paper P so that it can be easily removed from the belt 10′ is provided at a position facing the roller 13′ at the downstream end of the upper belt of the belt 10′.
A non-contact type of deelectrification unit 80 that uses corona discharge wire in order to deelectrify the belt 10′ is provided at the lower belt of the belt 10′. A cleaning blade 61a′ which can be moved towards or away from the roller 12′ via the belt 10′ is also provided in the roller 12′ portion. This blade 61a′ is moved so that it can avoid the joints in the belt 10′.
The image forming units 14BK′, 14C′, 14M′, and 14Y′ provided around the belt 10′, the optical writing unit 18′, the transfer apparatuses 73BK, 73C, 73M, and 73Y, the static electrifier 78, the cleaning blade 61a′, the deelectrification unit 79 and 80, and the like are means for executing the image formation processing.
In this image forming apparatus, the image forming is carried out in the following manner. When each of the photoconductor drums 71Y, 71M, 71C, and 71BK begin to rotate, the photoconductor drums are uniformly electrified in darkness during the rotation by the electrostatic devices 72Y, 72M, 72C, and 72BK. Exposure light Lb is then irradiated and scanned onto the exposure section with the writing timing shifted, and a latent image corresponding to the image to be created is formed. Toner images are then formed by the developing apparatuses 74Y, 74M, 74C, and 74BK so as to be transferred on top of each other on the same paper P.
The paper P stored in a paper supply section is fed out by the paper feed rollers 76. This paper then passes along the transporting path shown by the broken line and is temporarily stopped at the position of the pair of resistance rollers 75. The paper then waits for a time at which it can be fed out so as to match up with the toner images on the photoconductor drums 71Y, 71M, 71C, and 71BK at the transfer section. When the time arrives, the paper P that had been stopped by the resistance rollers 75 is fed out from the resistance rollers and is transported while being suctioned to the belt 10′ by the static electrifier 78. At this time, the belt 10′ is controlled by the marks or the like such that the paper is not placed on top of the joints in the belt 10′. Consequently, each of the toner images on the photoconductor drums are sequentially transferred onto paper S in the transfer sections where the paper makes contact with each of the photoconductors. The colors are thus superposed and a full color toner image is produced.
The positions where each of the photoconductor drums 71Y, 71M, 71C, and 71BK comes into contact with the belt 10′ form transfer sections and each of the transfer apparatuses 73Y, 73M, 73C, and 73BK are located in these transfer sections.
The paper P onto which the full color toner image has been transferred is deelectrified by the deelectrifying unit 79 and is then separated from the belt 10′. It is then fed as it is to the fixing apparatus 50′ where fixing is performed and is discharged onto the-paper discharge tray 81.
The residual toner remaining on the photoconductor drums 71Y, 71M, 71C, and 71BK reaches the cleaning units 1Y, 1M, 1C, and 1BK due to the rotation of the photoconductor drums, and the photoconductor drums 71Y, 71M, 71C, and 71BK are cleaned as they pass the cleaning units so as to be ready for the formation of the next image. After the paper P has been separated from the belt 10′, the belt 10′ is deelectrified by the deelectrifying unit 80. It then arrives at the cleaning blade 61a′ serving as cleaning means where it is cleaned and prepared for the transporting of the next paper.
The cleaning of the belt 10′ by the cleaning blade 61a′ is performed because a portion of the toner images from the photoconductor drums 71Y, 71M, 71C, and 71BK is transferred onto the belt 10′ and also because paper dust from the paper sticks to the belt 10′ and the cleaning is performed in order to prevent this transferred toner and paper dust and the like from contaminating the next paper.
In a tandem type of color image format, the downstream side roller 13′ is made the driver roller for the belt 10′ in order to tension the upper belt on which the paper P has been placed and a drive source MO2 is linked to the drive roller 13′.
Here, the cleaning blade 61a′ has such a construction that it can be moved towards or away from the roller 12′ via the belt 10′ and imparts a rotation load variation to the roller towards which and away from which it is moved. The load variation affecting the roller 12 as a result of the movement of the cleaning blade 61a′ is large enough to temporarily alter the tension on the belt 10′ for the same reason that applies to the cleaning blade 61a in the above described intermediate transfer type image forming apparatus.
If the cleaning blade 61a′ is moving towards or away from the belt 10′ when the photoconductor drums 71Y, 71M, 71C, and 71BK are transferring a toner image onto the paper P on the belt 10′, the slaver roller 12′ is directly affected by this load variation and the tension on the belt 10′ changes.
On the other hand, because the rotation speeds of the photoconductor drums 71Y, 71M, 71C, and 71BK are constant, the relative speed between the belt and the periphery of the photoconductor drum temporarily changes due to the variations in the belt tension, and it has been determined that color misregistration arises in the transferred toner image and pitch unevenness is generated.